occasionally adhere by means of their fibrous epidermis.
The species of this genus are chiefly distributed through the seas of the tropics, where they are very numerous. We have three or four on our own shores, the largest of which is commonly known as the Ark of Noah, though the species properly so named (Arca Noæ) is a native of the Mediterranean. Ours is the Arca tetragona of conchologists. The valves are deep, four-sided, and strongly angular, covered with fine radiating lines, of a warm reddish-brown hue, becoming paler and sometimes white on the front part. The valves sometimes attain a breadth of two inches, with a length of about three-fourths of an inch. It is found commonly all round the British coasts, most abundantly in the south-west, and in the extreme north. "It is taken in crevices of rocks, in chinks of old shells, in the interior of dead shells, and sometimes quite free. When found in con-